Please can someone help- I'm a British passport holder and would like to visit Wat Pah Nanachat for a few months... maybe longer.

Visa
As a British passport holder I'm 'visa exempt' unless I wish to stay longer than 3O days. So before I leave should I go here (link below) and apply for a 60 day tourist visa? just so it's less hassle when I'm there. Can I do this on arrival to Thailand?

Once I'm there, as I understand it, I can keep extending my Visa (by 15 days?) by popping across the border to Laos and Cambodia.

Single or return.
Again, this is only as I understand it: The airline that you travel with can get fined for bringing in travelers that don't have any plans of onward travel, so they require proof before they let you board in the UK. Now, if I bought a single with no proof of onward travel (only with the excuse that I want to travel around Asia- which I don't) would this be okay?

The difficult bit seems to be boarding the plane in the UK because on arrival to Thailand customs only require proof that I have adequate funds to provide for onward travel- like a bank statement. Is this right?

I keep looking on skyscanner but only end up in confusion so really appreciate any help

I would think that probably best to get for as long a Visa as you can before you go. Doing paperwork in Thailand can be a long and arduous process...

I'm a little confused about your question about tickets. Depending on where you are going from/to, a return long-haul ticket is often much cheaper than two one-way tickets, even if you need to pay for changing the dates later.

I flew to BKK this morning, and nobody asked to see my onward ticket at any stage. Better to be safe than sorry however.
Arrange your visa in advance, as sometimes the visa-exempt visa you can get at the airport is NOT EXTENDABLE by simply crossing the border.

Then,
saturated with joy,
you will put an end to suffering and stress.SN 9.11

I've been to thailand a dozen times and i've never been asked for proof of an onward ticket, and only proof of funds if applying for a longer visa beforehand.

For renewal you'll probably have to cross the border into Laos or Cambodia to get another 60 day visa, at WPN they'll know the best means.

“Peace is within oneself to be found in the same place as agitation and suffering. It is not found in a forest or on a hilltop, nor is it given by a teacher. Where you experience suffering, you can also find freedom from suffering. Trying to run away from suffering is actually to run toward it.” ― Ajahn Chah

I've also flown one way to Thailand many times, and have never been bothered at the ticket counter. Best I know, you'll have to leave after 30 days, unless you can arrange a work visa, which is not worth the effort.
Like many, I've done the famous visa run to Cambodia. Got tired of it, and now I go to Singapore. Either way its a pointless hassle. Sure wish Thailand would change the rules.

the already-mentioned thaivisa site is the go-to place for the best information. I have been denied boarding twice in Canada for flights to Thailand. These events occurred many years ago; but if you check thaivisa.com you will see that people report such incidents on a regular basis.
On both occasions I was able to persuade the airline managers to let me board. Once it was because I had a double entry Thai visa in my passport. So I would suggest you get a visa before departing.
It doesn't matter what the reality is upon arrival in Thailand... i.e. they don't care if you have a return ticket. What matters to the airlines is that they can be held responsible for returning you if you are denied entry. The rules are written that way; and they have no idea how, or if, the the Thais enforce them.